Determination of H-3 and C-14 in the frame of decommissioning projects at the Paul Scherrer Institute

Transcription

1 Determination of H-3 and C-14 in the frame of decommissioning projects at the Paul Scherrer Institute Jost Eikenberg, Maya Jäggi, Max Rüthi Paul Scherrer Institute, CH-5232 Villigen, Switzerland

2 Content of this presentation 1. Introduction to the decommissioning project 2. Principles of gaseous H-3 and C-14 separation 3. LSC measurements and activity calculations

3 real graph of PSI + ZWILG

4 Proton beam line: injector II, acceleration of protons to km/s (80% of the speed of light)

5

6 Decommissioned nuclear fission research reactors at PSI Graphite reflector nuclear power reactor (source for C-14) designed 1955 by Prof. Paul Scherrer Heavy water swimming pool reactor (source for H-3) constructed 1962

7 Sample types in the waste tank from the decommissioning activities Slurry mud materials consisting of various components (e.g. organics, concrete powder with BaSO 4 acting as neutron shielding material, graphite powder, clay minerals etc.) Water samples, that turned to be highly quenched when mixed with scintillation cocktail

8 types of spectrometers for radioisotope specific determination of the radwaste slurry counting technique gamma-spectrometry liquid scintillation spectrometry (LSC) alpha-spectrometry radioisotopes 60 Co, 137 Cs, 133 Ba, 152,154 Eu 3 H, 14 C, 55 Fe, 63 i, 89,90 Sr, 241 Pu U,238 Pu, Pu, 241 m, 244 Cm

9 Procedures for measuring H-3 and C-14 in the waste material water samples H-3: filtration followed by direct measurement: 10 ml water, 10 ml Ultima Gold LLT, window (low energy) C-14: same cocktail, window B (intermediate beta energy mud samples after drying and combustion H-3: bubbling through water columns (ph 3), cocktail composition as for direct measurement C-14: converting into carbonate ion (ph 12) using water bubblers, cocktail comp. 5:15 (high alkaline solution)

10 Extraction line system

11 Summarizing the principle of the separation procedure The dried slurry is combusted/decomposed in a ring oven in closed tube system The released gaseous components 1 H 3 HO and 14 CO 2 are routed with nitrogen gas towards a water bubbler system and then consecutively washed out Water samles are taken and subsequently analysed via LSC

12 Hydrolysis and dissociation of carbonic acid 1. Hydrolysis: the H 2 CO 3 /CO 2 equilibrium system H + 2CO3 CO2 H 2O H (1) 2. Dissociation to the carbonate ion via two dissociation steps in neutral and alkaline media CO HCO + H + ( pk = ) HCO CO 2 + H + ( pk = )

13 Speciation of carbonic acid as a function of ph distribution of species [%] H 2 CO 3 HCO - 3 pk 1 =6.5 CO 2-3 pk 2 = ph

14 Calculation of the trapping factors for 3 H and 14 C 1, cor [ Bq / l] = 1, m f (1) [ Bq / l] = f 2, m 1, m ( 1 ) (2) f = 1, m 1, m 2, m (3)

15 LSC-counter with BGOcoincidenceanticoincidence background suppression

16 β-continuum-spectra: 3 H, 14 C 1200 LSC: spectrum of 14 C und 3 H H 14 C intensity (not to scale) E max =18.6 kev E max = 156 kev energy (kev)

17 general relationship between activity and counting rate I = = t ε ε Example H-3, C-14, two window counting window : H-3 + C-14, window B: C-14 cpm cpm 3) = cpm, net ( ) C. cpmb ( C 14) cmp ( H 14 B

18 Determination of the counting efficiency (a) by adding a spike of the same isotope ε = ε i = i, afteraddition i, added i, beforeaddition (b) utomatically via quench curve correction

19 3 H, 14 C quench curves: efficiency in dependency of the spectral index from the external standard 60 % efficiency C 3 H tsie

20 Results: reference date water samples H-3: 60 kbq/l 2σ uncertainty < 10% mud samples H-3: 1-10 kbq/kg 2σ uncertainty < 15% C-14: < 0.1 kbq/l C-14: 1-10 kbq/kg 2σ uncertainty < 10%

21 Calculating sample activity and overall uncertainty First step: calculate sample activity ε i = spiked, net i, added sample = sample, net ε i Second step: calculate type uncertainties resulting from counting statistics sample : t t s = s + s = ( t 0) ± + T t 0 0 spiked, tot spiked sample : spiked, net = spiked, net + spiked = ( spiked, tot s ) ± + tspiked, tot t s s

22 Third step: calculate the relative uncertainty of the counting efficiency ε i = spiked, net i, added ε = ε spiked spiked, net 2 + i, added i, added 2 Fourth step: Calculate the relative activity uncertainty of the sample sample = sample, net ε i sample sample = sample sample, net 2 + ε i ε i 2

23 Conclusions The isotopes C-14 and H-3 are of major concern in waste material from decommissioned reactors in Switzerland Separation between both isotopes is easily performed using a closed decomposition system with consecutive wash-out of the components HTO and 12,14 CO 2. Liquid scintillation counting with addition of internal standardisation isotopes ( 3 H, 14 C) yields highly precise results